1. Field of the Invention
The present invention relates to an intrusion warning system utilizing an antenna on which an A.C. voltage is impressed.
2. Description of the Prior Art
FIG. 1(a) is a block diagram showing an example of the conventional intrusion warning system utilizing an electric field, such as shown in the U.S. Pat. No. 4,064,499. In the system of FIG. 1(a), a voltage of a high frequency, for example, about 9750 Hz shown by FIG. 2(E) and generated by a signal generating circuit E is applied across a field wire S and the ground, and a voltage change induced on an antenna A disposed with a substantially uniform distance gap g from the field wire S is detected and processed to produce an intruder detecting signal. The voltage change due to the intruder is detected as follows:
Provided that a resistance between the antenna and the ground, i.e. an input impedance of the amplifier is sufficiently large, by considering the equivalent circuit of FIG. 1(b), the induced voltage V, for the case that there is no intruder, is given by: ##EQU1## wherein C.sub.1 ' . . . is a capacitance between the field wire S and antenna A,
C.sub.o . . . is a capacitance between the antenna A and the ground, PA1 e . . . is a voltage of the high frequency signal applied to the field wire S.
Then, by considering the intruder's body as an electric conductor, and capacitance between the intruder's body and the antenna A is C.sub.M, the induced voltage V.sub.a ' becomes as shown by the following equation (2): ##EQU2##
The induced voltage is amplified by an amplifier 7, then detected by a detection after circuit 8. After passing a band-pass filter 9 the voltage is provided to a threshold circuit 10, where its input value V.sub.a is compared with a predetermined threshold value V.sub.th.
When a man passes through the electric field of the antenna A as shown by FIG. 1(c), the induced voltage V.sub.a ' changes as shown by FIG. 2(V.sub.a ') by the change of capacitance C.sub.M, and therefore, the input voltage V.sub.a to the threshold circuit 10 changes as shown by FIG. 2(V.sub.a) and by FIG. 1(d). Therefore, when the input voltage V.sub.a becomes lower than the predetermined threshold value V.sub.thL at the time t.sub.1 as shown by FIG. 2(V.sub.a), the threshold circuit 10 sends an output signal to the warning circuit 6, which issues a warning signal at the time t.sub.1 as shown by FIG. 2(V.sub.s), to light a lamp or ring a buzzer.
Such a conventional intruder detection system uses two wires S and A, and therefore when a strong wind blows, the gap between the two wire is likely to change considerably, thereby producing undesirable change of the capacitance C.sub.1 between the two wires, and therefore, false alarms are likely to be issued. Furthermore, when the area to be protected is broad, it is expensive to install two wires with a uniform gap and also is sometimes difficult due to the contours of the land.
Furthermore, the sensitivity of the detection is greatly influenced by changes of the capacitance C.sub.o between the antenna wire S and the ground which is dependent on the length of the antenna wire S and height of the antenna wire S from the ground. Therefore, the conventional system has had a problem that the apparatus can not be as sensitive as designed, since the design is made for an average antenna wire of average height and length, or in other words, the apparatus is useful only when the antenna is in a limited range of height and length. Moreover providing a pair of wires may be difficult in practice because of surrounding conditions.
In addition, the conventional system has a further problem that false alarms are likely to be caused by interference or beating between the high frequency signal fed to the antenna and a high harmonic of the frequency of the A.C. power existing in the protected area. Such beating produces a signal of a very low frequency such as 0.1 to 2 Hz and such low frequency signal passes the band-pass filter 9 and causes the alarm circuit 6 to produce a false alarm. In order to avoid such false warning, in U.S. Pat. No. 4,064,499, the oscillation frequency of the signal to be fed to the antenna is locked by a complicated frequency lock circuit or the alarm output is disabled by utilizing an output of a beat frequency detector circuit (BFD) when the A.C. power frequency produces the abovementioned undesirable beating. However, if the first measure, using the frequency lock circuit (FLC) is employed, the increased complication of the circuitry increases the cost of the system and if the second measure, disabling the alarm, is employed intruder detection is often disabled.